The present invention relates to machines for molding plastics, and more particularly to rotational molding machines wherein the molding process is carried out within a heated over, and the mold is rotated about two axes under controlled conditions.
Rotational molding machines are particularly adapted for the manufacture of large parts from vinyl plastisols. The particular materials which work well in conjunction with rotational molding machines include thermoplastic polyethylene powders, and particularly low density polyethylene. Other forms of thermoplastic powders which have been commercially used with rotational molding machines include low and high density polyethylene, polyethylene copolymers, cellulose acetate-butyrate, vinyl dry blends, impact styrenes, and high-modulate thermoplastics. These materials are introduced into a mold in either a heat convertible liquid or a powder, and the rotational molding process changes the physical form of the material to a continuous solid. This change occurs while the mold is rotationally moved about two perpendicular axes while being held within the machine.
A rotational molding machine may be adapted to hold one or a plurality of molds during the rotational molding process, the molds being held into the machine by a frame work called a spider. A two-axis drive is imparted to the spider to cause the necessary rotational movement during the process.
Rotational molding machines are generally classified into batch-type machines or continuous machines. Continuous rotational molding machines typically are semi-automated or fully automated, having a plurality of driven spindles upon which the spiders are attached, and the path of travel of the spiders passes through various processing stations to permit loading, heating, cooling and stripping of the molds while the machine operates more or less continuously. The batch-type rotational molding machine, which is the subject of the present invention, typically utilizes a rotatable spider mounted within an oven, and the loading, heating, cooling and stripping process steps are usually manually performed while the spider is in a fixed position within the oven. In some cases the molds are moved manually from the oven to a cooling, unloading and filling station, wherein some of these operational process steps may be performed outside of the oven.
The manual handling steps required with batch rotational molding machines are time consuming and difficult to perform. In particular, the steps associated with loading and unloading the molds into the machine require considerable time, thereby adversely affecting the production rate of such machines. Accordingly, there is a need for a batch rotational molding machine which may be quickly loaded and unloaded, and which at the same time will facilitate the execution of the molding process steps required.